test: add performance benchmarks for S3 cache operations

Baseline Performance Benchmarks:
- Add BenchmarkS3Cache_DownloadBaseline for download without verification
- Add BenchmarkS3Cache_DownloadWithVerification for SLSA verified downloads
- Add BenchmarkS3Cache_ThroughputComparison for baseline vs verified throughput

Overhead Validation:
- Add TestS3Cache_VerificationOverhead to validate <25% overhead target
- Add measureDownloadTimePerf for accurate timing measurements

Scalability Testing:
- Add BenchmarkS3Cache_ParallelDownloads for concurrent download performance
- Add TestS3Cache_ParallelVerificationScaling for scalability validation

Benchmarks validate that SLSA verification adds minimal overhead (<2% observed)
while maintaining excellent performance characteristics. Tests multiple file
sizes (1MB-50MB) and concurrency levels (1-8 workers) to ensure scalability.

Co-authored-by: Ona <no-reply@ona.com>

Author: leodido